To date, organic EL devices including a plurality of emitting layers each of which emits light of a different wavelength are known. Such organic EL devices are also known to provide mixed-color light in which the lights emitted by the emitting layers are mixed together. One of such organic EL devices includes a layered red-emitting layer, green-emitting layer and blue-emitting layer, and provides white light in which emissions from the emitting layers are mixed together.
Excited states of organic compounds encompass a singlet state and a triplet state. Emission from the singlet state is called as fluorescence while emission from the triplet state is called as phosphorescence. Typically, the singlet state and the triplet state occur in a ratio of 1:3.
Conventional organic EL devices have mainly used fluorescent emitting materials for emission of fluorescence. In such an organic EL device that utilizes the fluorescent emission, only the singlet (i.e., 25% of the excitation generated in an emitting layer) contributes to the emission while the triplet (i.e., the remaining 75%) is deactivated without emitting.
In order to enhance the luminous efficiency of organic EL devices, developments are being made on phosphorescent emitting materials for emitting phosphorescence, i.e., emission from the triplet (e.g., Patent Document 1). For instance, reports have been made on red-emitting phosphorescent materials and green-emitting phosphorescent materials. It should be noted that no practically-applicable blue-emitting phosphorescent material has been obtained.
With application of the above phosphorescent materials, organic EL devices for mixed-color emission are capable of enhancing luminous efficiency.
For instance, a known organic EL device provides white emission with use of a blue-emitting fluorescent materials and a red-to-green-emitting phosphorescent materials. According to such a known device, enhancement of quantum efficiency in the red to green emission contributes to enhancement of the luminous efficiency of the entire organic EL device.
However, even in such an organic EL device for mixed-color emission, while emission is obtained from the triplet in the red to green emitting layers, the triplet in the blue emitting layer has been deactivated without contributing to emission.
In view of the above, proposals have been made on an organic EL device in which the triplet of the blue emitting layer is diffused in the red and green emitting layers so that red and green phosphorescence is obtained therefrom (e.g., Patent Documents 2 and 3, Non-Patent Document 1). According to such an organic EL device, the energy of the triplet in the blue emitting layer, which have been typically to be deactivated without contributing to emission, can be utilized for obtaining red and green phosphorescence. Thus, the luminous efficiency of the entire organic EL device can be enhanced.
The organic EL device disclosed in Non-Patent Document 1 includes a blue fluorescent-emitting layer, blocking layer, red phosphorescent-emitting layer, green phosphorescent-emitting layer, blocking layer and blue fluorescent-emitting layer in this order. According to this document, blue fluorescence is obtainable from the singlet in the blue fluorescent-emitting layers, and the triplet in the blue fluorescent-emitting layers is diffused in the red and green phosphorescent-emitting layers via the blocking layers. Then, the triplet in the red and green phosphorescent-emitting layers is generated, from which red and green phosphorescence is obtainable. According to the report, the blue fluorescence and the red and green phosphorescence are mixed together, thereby providing white emission as a whole.    Patent Document 1: US2002/182441    Patent Document 2: WO2006/038020    Patent Document 3: WO2004/060026    Non-Patent Document 1: nature vol 440 p. 908